The present invention relates to an emulsion for post-treating planographic printing plates prepared by electrophotographic means, in order to improve the print quality. The present invention also relates to a process for producing printing plates, employing the aforesaid emulsion.
Electrophotographic processes for producing printing plates are generally known to the art and are described, for example, in German Auslegeschrift No. 11 17 391 (equivalent to British Patent No. 944,126); German Auslegeschrift No. 23 22 047 (equivalent to British Patent No. 1,465,927); and German Patent No. 25 26 720 (equivalent to U.S. Pat. No. 4,063,948).
The basic principle of these electrophotographic methods comprises charging and imagewise exposing a photoconductor layer, which has been applied to an electrically conductive layer support; developing the image by means of dry or liquid toner; fixing the resulting toner image by heating; and decoating the printing plate by removing the unprotected, image-free areas of the photoconductor layer. The toner image areas of the resulting offset printing plate are ink-receptive, whereas the bared areas of the support surface are water-receptive.
It has been stated that--in particular in relatively large, coherent, full shade areas, but also in halftone areas--printing plates prepared in this way tend to have some places where the printing ink is transferred nonhomogeneously. A search for the cause of this undesirable phenomenon indicated that the toner, which usually contains a resin that is soluble in organic solvents, starts to swell when it remains in contact with the printing ink over a prolonged period. As a result the toner gets tacky and is able to retain dust and dirt particles, or may form an irregular, wrinkled surface.
U.S. Pat. No. 3,158,476 has disclosed a process for increasing the print run of fixed electrophotographic printing plates, the image-free areas of which are hydrophilic, by (1) coating the image-free, decoated areas with aqueous colloidal substances and, after the coating has dried, (2) treating the printing plate surface with a lacquer containing organic solvents. The laquer treatment dissolves the fixed toner image and the underlying photoconductive layer, and the lacquer firmly attaches to the layer support image only in these image areas. The plate is thereafter treated with water, whereby the coating is removed again from the image-free areas. It is not possible to use this process, however, to produce printing plates that meet present-day standards, for the screen dots are enlarged by the applied lacquer coating; as a consequence, the tone values of the original cannot be reproduced truly. Moreover, the described process is quite complicated and expensive.
It is also known to coat those areas of the layer support, which have been bared by the decoating step, with a solution containing (i) a film-forming polymer, which is soluble in water and insoluble in hydrocarbon solvents, and (ii) a vinylphosphonic acid compound. The solution is then allowed to dry, so that a uniform coating is obtained, and the toner is removed from the photoconductor layer by washing with a hydrocarbon mixture comprising an aromatic proportion of at least 20%.
This process requires a considerable number of different process steps for the production of a plate which is ready for printing. For example, the decoating step must be followed by rinsing, finishing and drying; thereafter, the toner-removing solution must be applied, rubbed in and removed with the aid of a blade, which step is followed by renewing rinsing with water, applying a finishing solution and drying. Seven different process steps are necessary to perform this process; this renders the printing plate production technically complicated, time-consuming and, thus, cost-intensive.